In preparation of semiconductor devices, a substrate comprising sapphire (Al2O3) or silicon carbide (SiC) is generally used to grow a GaN-based nitride layer. However, these substrates are different from the GaN-based nitride layer in material properties such as lattice constant and thermal expansion coefficient and thus, it is difficult to grow a high quality GaN-based nitride layer. There have been developed and disclosed many techniques for growing the GaN-based nitride layer.
One of the representative methods is using a buffer layer. According to this method, a Al(x)Ga(y)In(z)N film (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) is grown in a single or various composition at 450° C. to 600° C., followed by raising temperature to form the Al(x)Ga(y)In(z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) into a nuclei, which is used as a seed to grow a high quality GaN-based nitride layer. Also, a AlN buffer layer (Japanese Patent publication No. 62-119196), a LT-AlGaN buffer layer (U.S. Pat. No. 5,290,393/Japanese Patent publication No. 4-297023: 1991), a LT(low temperature)-AlGaInN buffer layer (U.S. Pat. No. 6,508,878), a LT-AlInN buffer layer and the like are used. However, when a GaN-based nitride layer is grown by these methods, the grown GaN-based nitride layer has a dislocation density of 1010 to 1012/cm2.
Also, unlike the above-described examples, in which a buffer layer is grown on a sapphire substrate at a low temperature, a GaN-based nitride layer may be directly grown on the substrate at a high temperature. However, there is much room for improvement.
FIG. 1 is a view to explain a method for forming a GaN-based nitride layer on a sapphire substrate according to the prior art, in which a GaN or AlGaN or InGaN buffer layer 11 is grown on a sapphire substrate 10 at a low temperature (typically 500° C. to 800° C.) and a GaN film 12 is grown at a high temperature of about 1050° C. In this case, the GaN or AlGaN or InGaN buffer layer 11 grown at a low temperature is deformed, when the temperature is raised, to have a 3-dimensional shape. The deformed buffer layer 11 serves to buffer strain between the GaN film 12 and the sapphire substrate 10. However, fundamentally, the GaN or AlGaN or InGaN buffer layer 11 also has a big lattice mismatch with the lattice of the sapphire substrate 10 and induces lattice mismatch in the GaN film 12 grown thereon (lattice dislocation density: 1010 to 1012/cm2).
Therefore, in order to reduce such lattice mismatch and improve properties of a thin film to be grown, it is preferable to use a substrate having a lattice corresponding to that of GaN, instead of the sapphire substrate which has been commonly used. However, it is matter-of-fact impossible.
A group where the present inventors also belong to has found that when a buffer layer of a SiC thin film, instead of Al(x)Ga(y)In(z)N (0≦x≦1, 0≦y≦1 ,0≦z≦1, x+y+z=1), which has been disclosed, is formed on a sapphire substrate and a GaN-based nitride layer is formed thereon, the formed GaN-based nitride layer has excellent properties and filed an application based on this discovery (Korean Patent Application Nos. 10-2003-52936 and 10-2003-85334).